Regioselective Mannich bases of pyrrole-2-carbaldehyde and binuclear copper(II) complexes of bis(iminopyrrolyl) ligand containing the piperazine ring

Article abstract of DOI:10.1016/j.ica.2016.02.023

Iminopyrrolyl ligands have attracted an attention in the field of coordination chemistry. As every iminopyrrolyl ligand contains at least one acidic pyrrole NH group, it often forms an anionic five-membered chelate ring with a metal. Unlike the familiar pincer framework ligand systems, iminopyrrolyl coordination chemistry is in a developing stage. Hence, we set the synthesis of bis(iminopyrrole) framework ligand containing the piperazine ring as a spacer unit between two mono(iminopyrrolyl) moieties. The Mannich reaction of pyrrole-2-carbaldehyde with piperazine and formaldehyde afforded two new dialdehydes: N,N′-bis(5-formylpyrrol-1-ylmethyl)piperazine 1 and N,N′-bis(5-formylpyrrol-2-ylmethyl)piperazine, 2, which are regioselective products formed in good yields. The pyrrole N aminomethylated dialdehyde 1 was isolated in the absence of an added acid. Conversely, the pyrrole α-C aminomethylated dialdehyde compound 2 was obtained in the presence of a mineral acid. They have different spectroscopic and physical properties. Even they offered different Schiff bases under the same reaction conditions. The Schiff base condensation reaction of 2 with 2,6-diisopropylanilline in the presence of HNO₃ gave the expected [1+2] Schiff base in 53% yield after column separation. Conversely, the other dialdehyde 1 yielded the piperazinylmethyl C-N bonds hydrolyzed known 2-iminopyrrole compound. Further, the treatment of the structurally characterized [1+2] bis(iminopyrrolyl) ligand with copper(II) carboxylates [Cu(OOCR)₂(H₂O)] (R = H, Me and Ph) gave three different binuclear five-coordinate copper(II) complexes. Their structures were determined by single crystal X-ray diffraction studies. Interestingly, the piperazine ring nitrogens are not coordinated, rather involved in intramolecular hydrogen bonding interactions with the coordinated solvent molecules at the copper atom.

Full text of DOI:10.1016/j.ica.2016.02.023

Inorganica Chimica Acta 445 (2016) 70–78
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Regioselective Mannich bases of pyrrole-2-carbaldehyde and binuclear
copper(II) complexes of bis(iminopyrrolyl) ligand containing
the piperazine ring
⇑
Rajnish Kumar, Tapas Paul, Oishi Jana, Ganesan Mani
Department of Chemistry, Indian Institute of Technology-Kharagpur, Kharagpur, West Bengal 721 302, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 August 2015
Received in revised form 3 December 2015
Accepted 12 February 2016
Available online 21 February 2016
Iminopyrrolyl ligands have attracted an attention in the field of coordination chemistry. As every
iminopyrrolyl ligand contains at least one acidic pyrrole NH group, it often forms an anionic five-mem-
bered chelate ring with a metal. Unlike the familiar pincer framework ligand systems, iminopyrrolyl coor-
dination chemistry is in a developing stage. Hence, we set the synthesis of bis(iminopyrrole) framework
ligand containing the piperazine ring as a spacer unit between two mono(iminopyrrolyl) moieties. The
Mannich reaction of pyrrole-2-carbaldehyde with piperazine and formaldehyde afforded two new dialde-
hydes: N,N -bis(5-formylpyrrol-1-ylmethyl)piperazine 1 and N,N -bis(5-formylpyrrol-2-ylmethyl)piper-
azine, 2, which are regioselective products formed in good yields. The pyrrole N aminomethylated
dialdehyde 1 was isolated in the absence of an added acid. Conversely, the pyrrole a-C aminomethylated
dialdehyde compound 2 was obtained in the presence of a mineral acid. They have different spectroscopic
and physical properties. Even they offered different Schiff bases under the same reaction conditions. The
Schiff base condensation reaction of 2 with 2,6-diisopropylanilline in the presence of HNO3 gave the
expected [1+2] Schiff base in 53% yield after column separation. Conversely, the other dialdehyde 1
yielded the piperazinylmethyl C–N bonds hydrolyzed known 2-iminopyrrole compound. Further, the
treatment of the structurally characterized [1+2] bis(iminopyrrolyl) ligand with copper(II) carboxylates
Keywords:
Copper(II)
Bis(iminopyrrolyl)
Mannich reaction
Regioselectivity
Schiff base
0
0
Hydrogen bonding
2 2
[Cu(OOCR) (H O)] (R = H, Me and Ph) gave three different binuclear five-coordinate copper(II) complexes.
Their structures were determined by single crystal X-ray diffraction studies. Interestingly, the piperazine
ring nitrogens are not coordinated, rather involved in intramolecular hydrogen bonding interactions with
the coordinated solvent molecules at the copper atom.
Ó 2016 Elsevier B.V. All rights reserved.
1
. Introduction
The mono(iminopyrrolyl) and bis(iminopyrrolyl) ligand sys-
analogous bis(imino)dipyrromethanes and their coordination
chemistry [6]. In addition, one of the noticeable features of metal
complexes containing pyrrole based ligands is the binding mode
of the pyrrolidine ring; it coordinates primarily in two modes:
tems possess an attractive coordination motif with metals, which
drives their metal complexes synthesis [1]. Iminopyrrolyl ligands
are conveniently synthesized by condensation reactions of the cor-
responding pyrrole aldehyde with a variety of primary amines,
which have led to a variety of iminopyrrolyl ligands having varying
steric and electronic properties [2]. In addition, ligands containing
two iminopyrrole moieties linked through some aromatic or ali-
phatic spacer unit have also been developed [3]. Iminopyrrolyl
metal complexes have been used as catalysts for ethylene poly-
1
5
1
j
N (terminal) and
g /j N (bridging), which facilitate multinuclear
complex formation [7].
Gmeiner and co-workers have reported the regioselective
formylation of the piperazinylmethyl substituted pyrrole [8].
Keypour et al. have reported several Schiff bases containing the
piperazine ring and studied their coordination chemistry [9]. In
addition, Lee, Huang and co-workers have recently reported the
mono- and dipiperazinylmethyl substituted pyrrole by the
Mannich reactions of pyrrole [10]. We are interested in developing
metal complexes of pyrrole-based ligand systems and their cat-
alytic applications. Recently, we reported a few palladium com-
plexes containing bis(iminopyrrolylmethyl)amine ligand, which
shows an interesting amine–azafulvene tautomeric structure upon
merization [4] and the ring opening polymerization of
tone [5] reactions. Similarly, a large progress has been made on the
e-caprolac-
⇑
Corresponding author. Fax: +91 3222 282252.
E-mail address: gmani@chem.iitkgp.ernet.in (G. Mani).
http://dx.doi.org/10.1016/j.ica.2016.02.023
020-1693/Ó 2016 Elsevier B.V. All rights reserved.
0

R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
71
complex formation, and their Suzuki cross coupling reactions [11].
These interesting properties of iminopyrrolyl ligands have made us
to become interested in designing different iminopyrrolyl ligands
for metal complexes study. Herein, we report two new regioselec-
tively formed dialdehydes containing the piperazine ring as a
spacer unit via the Mannich reaction of pyrrole-2-carbaldehyde.
In addition, we also report synthesis and structural characteriza-
tion of binuclear copper(II) complexes containing a new bis
X-ray diffraction analysis. The X-ray structure of 2 and the selected
bond lengths and angles are given in Fig. 1 and Table 1, respec-
tively. The two pyrrolyl moieties are bound in the equatorial posi-
tions of the chair conformation of the piperazine ring. While one of
the pyrrolealdehyde moieties with its carbonyl O and the pyrrolic
NH groups is oriented in one direction, the same groups in the
other side of the molecule are oriented in an opposite direction.
This results in four intermolecular hydrogen bonds between these
groups for every molecule and formation of the 1D supramolecular
chain structure in the crystal lattice (Fig. 1c). This is similar to the
(
iminopyrrolyl) ligand derived from the dialdehyde molecule.
hydrogen bonding observed in the structure of N,N-di(
a-formyl-
2
. Results and discussion
pyrrolyl- -methyl)-N-methylamine reported by us [13].
a
2.1. Synthesis and characterization of dialdehydes
2.2. Synthesis and characterization of bis(iminopyrrolyl) ligand
Pyrrole-2-carbaldehyde reacts with a mixture of piperazine and
formaldehyde in 2:1:2 molar ratios, respectively, to give the pyr-
role N aminomethylated dialdehyde compound 1, which was iso-
lated as a colorless solid from the reaction mixture without
further purification in 71% yield. Interestingly, when the same
reaction was carried out in the presence of an acid, the pyrrole
Having synthesized the dialdehyde compounds containing the
piperazine ring, we then set the synthesis of their open chain Schiff
bases because such bases would provide two sets of donor atoms
to a metal atom and give bimetallic complexes. The Schiff base
condensation reaction of 2 with two equiv of 2,6-diisopropylani-
a-C aminomethylated dialdehyde compound 2 was obtained in
3
line in the presence of two equiv of HNO gave the [1+2] conden-
almost the same yield as 1 (Scheme 1). Although the HRMS spectra
sation product 3 and the partially hydrolyzed [1+1] product 4 in
53% and 20% yields, respectively, after column chromatographic
separation (Scheme 2). Conversely, the analogous reaction using
1 resulted in the formation of the monoiminopyrrole compound
5 owing to the hydrolysis of the pyrrolic N–C bond on either side
of molecule by nitric acid. While the compound 5 is a known com-
pound [2b], compounds 3 and 4 are new compounds and charac-
terized by spectroscopic methods.
of 1 and 2 showed the same molecular ion peak at m/z 301.1626
+
(
calc. mass 301.1665) corresponding to their [M+H ] ions, these
regioselectively formed products could be differentiated by NMR
1
and IR methods. In the H NMR spectrum of 1 in CDCl
ring CH resonances appeared as two multiplets at d = 6.95 and
.23 ppm with their integrated intensity ratio of 2:1. In contrast,
3
, the pyrrole
6
compound 2 showed the 1:1 integrated intensity ratio for the pyr-
role ring CH protons which appeared as multiplets at d = 6.89 and
The 1H NMR spectrum of 3 features a sharp singlet at
d = 7.85 ppm for the azomethine protons and at d = 3.65 ppm for
the methylene protons. The pyrrolic NH resonance was not
observed owing to an amine-azafulvene tautomeric structure,
which forms upon transfer of the pyrrolic NH proton to the basic
imine nitrogen atom. Analogous behavior has been reported for
the other iminopyrrole molecules reported by us [11]. The FT-IR
6
.16 ppm; its pyrrolic NH resonance appeared as broad singlet at
d = 9.61 ppm. Further, the N-bound methylene groups in 1 res-
onates as a sharp singlet at d = 5.15 ppm, which is shifted down-
field by 1.59 ppm as compared to the resonance of the same
group in 2 appearing at d = 3.56 ppm. Furthermore, this regioselec-
tivity is supported by DEPT-135 spectra, which showed three sig-
nals for 1 and two signals for 2 for their pyrrole ring carbon
ꢀ1
spectrum shows the m(CH@N) stretching frequency at 1627 cm .
+
atoms. Furthermore, the FT-IR spectrum of 2 displays the
m
(NH)
In addition, the molecular ion peak [M+H ] at m/z 619.4512 (calc.
ꢀ1
stretching frequency at 3239 cm appearing as a strong band,
which is absent in the IR spectrum of 1. These two compounds
exhibit different physical properties; compound 1 is highly soluble
mass 619.4488) is observed in the HRMS spectrum. Conversely,
1
the H NMR spectrum of 4 features a broad resonance at
d = 9.82 ppm which is assigned to the pyrrolic NH group located
adjacent to the aldehyde group, while the other NH resonance does
not appear owing to the amine–azafulvene tautomerism. The other
resonances and their integrated intensity ratios are in accord with
the structure of 4. Besides, compound 4 shows the molecular ion
peak at m/z 460.3111 (calc. mass 460.3076) corresponding to the
in common organic solvents such as MeOH, CHCl
whereas compound 2 is poorly soluble in these solvents. The pyr-
rolic N versus -C regioselectivity could be explained in terms of
3
, DCM, etc.,
a
acidity of the pyrrole-2-carbaldehyde NH group. In the absence
of an added acid, the base-catalyzed Mannich reaction mechanism
operates [12]. Thus, the piperazine probably abstracts the pyrrolic
NH proton and resulting pyrrolidine anion attacks the in situ
generated iminium ions present in the mixture to give the
N-aminomethylated product. Conversely, in the case of acidic
+
mass of its [M+H ] ion in the HRMS spectrum.
The structure of the [1+2] Schiff base 3 was confirmed by the
single crystal X-ray diffraction method (Fig. 2). The X-ray structure
revealed the typical iminopyrrole form rather than the amine–
azafulvene tautomeric structure, as shown by the N1–C13 (1.274
(3) Å) and C13–C14 (1.432(4) Å) distances, corresponding to the
double and single bonds, respectively. The molecule possesses
two binding cavities for metal atoms. There is no bound water in
medium, the pyrrolic
to give the expected
a
-C is active and attacks the iminium ion
a
-C aminomethylated product.
While our attempts to get suitable single crystals of compound
1
have failed, the structure of 2 was confirmed by single crystal
H
O
H
N
(
(
i) 2 HCl/MeOH/
O
MeOH/H O
2
HN
NH
H
H O/r.t./12 h
2
N
N
N
H
HCHO
N
RT, 12 h
H
O
ii) 2 NaOH/
NH
HN
30 min
N
N
O
H
O
1, 71%
2, 71%
Scheme 1. Synthesis of the regioselective dialdehydes 1 and 2 containing the piperazine ring.

7
2
R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
0
Fig. 1. The X-ray crystal structure of the N,N -bis(5-formylpyrrol-2-ylmethyl)piperazine, 2: (a) side view showing the e,e configuration/chair conformation and (b) the 1D
supramolecular chain structure formed by the intermolecular H-bonds. Symmetry transformations used to generate equivalent atoms: (i) ꢀx, ꢀy + 1, ꢀz + 1 and (ii) ꢀx ꢀ 1,
ꢀ
y + 1, ꢀz.
the cavity or solvent of crystallization in the lattice, which is a
desirable property for metal complexation reactions that require
moisture free conditions.
occupied by one of the oxygen atoms of the anisobidentate chelat-
ing formate anion.
The Addison geometric parameters
= b ꢀ
copper atom ( /60, where b and
angles; = 1 for a trigonal bipyramid and s
5
s
for each five-coordinate
are the two largest
= 0 for a square
s
a
a
2.3. Synthesis and characterization of the binuclear copper(II)
s
5
complexes
pyramidal structure) [14] are 0.18 (6), 0.38 (7) and 0.45 (8), indi-
cating their distorted geometries; the distortion is more for 8
towards a trigonal bipyramid. Each complex contains two mean
basal planes formed by the coordinated N and O donor atoms,
which are parallel to each other owing to symmetry, and the dis-
tance between them is 1.703 Å for 6, 2.453 Å for 7 and 1.220 Å for
8. The copper atom is located 0.102 Å for the formate complex 6,
0.282 Å for the acetate complex 7 and 0.304 Å for the benzoate
complex 8 above the mean basal plane towards the apical donor
atom. The Cu–Npyrrolide and the Cu–Nimine bond distances range
from 1.944(3) to 1.964(3) Å and 2.018(4) to 2.051(3) Å, respec-
tively, which are closer to the values in copper complexes con-
taining pyrrole-Schiff base ligands [1h,15]. The basal carboxylate
Cu–O distances range from 1.923(2) to 2.013(3) Å which are
also closer to the reported values [16].
The addition of one equiv of Schiff base 3 to a solution or sus-
pension of two equiv of copper carboxylates [Cu(OOCR) (H O)]
R = H, Me and Ph) in methanol results in an immediate color
change to green, which afforded the dianionic form of the bis
iminopyrrolyl) ligand 3 bridged binuclear copper(II) complexes
, 7 and 8 in good crystalline yields (Scheme 3). These complexes
2
2
(
(
6
contain two ‘well-separated’ copper atoms and are formed via
cleavage of the carboxylate ion bridged binuclear copper
complexes [Cu(OOCR) (H O) ], in which the two copper atoms lie
4 2 2
closer, and deprotonation of the pyrrolic NH groups of ligand by
the carboxylate anions, resulting in the formation of the corre-
sponding carboxylic acids. The FT-IR spectra of these complexes
show that the m(C@N) stretching frequencies shifted to lower wave
numbers as compared to that of the free ligand, indicating that the
azomethine nitrogen is bound to the copper atom.
In these structures, both inter- and intramolecular hydrogen
bonding interactions are present and their metric parameters
are given in Table 1. In the formate complex 6, the two bound
water molecules, one on each copper atom, form hydrogen bonds
with the piperazine nitrogen atoms as hydrogen bond acceptors
(Fig. 3). Besides, there are several intermolecular hydrogen bonds
formed between the lattice MeOH molecules, coordinated water
and carboxylate groups in the crystal lattice. In the case of
acetate 7 (Fig. 4) and benzoate 8 (Fig. 5) complexes, as the struc-
tures contain four coordinated methanol molecules, each one
forms one hydrogen bond with the adjacent hydrogen bond
acceptors. While the apical methanol OH groups form hydrogen
bonds with the spectator benzoate oxygen atoms, the basal
methanol OH groups form the same with the piperazine nitrogen
atoms. In total, the structure contains four intramolecular
hydrogen bonds.
The structure of complexes 6–8, given in Figs. 3–5, respectively,
was determined by single crystal X-ray diffraction studies and
their bond distances and angles are given in Table 1. It revealed
that each copper atom is five-coordinate and the geometry around
the copper atom is a distorted square pyramid with their axial
bond distances (2.691(4) Å, 2.258(3) Å and 2.296(2) Å) being
longer than the corresponding basal bond distances (1.944(4) Å,
2
.013(3) Å and 1.997(2) Å) in 6, 7 and 8, respectively. However,
these complexes differ by the constituent groups bonded to the
copper atom. The acetate 7 and benzoate 8 complexes contain
two methanol molecules: one in the basal and the other axial posi-
tions with the acetate and benzoate anions being coordinated in a
1
monodentate
plex, one H
j
-O fashion. Conversely, in the case of formate com-
2
O is bonded in the basal plane; the axial position is

R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
73
Table 1
3
. Conclusions
Selected bond lengths (Å) and angles (°) for 2, 3 and 6–8.
2
The regioselectivity, N versus
a
-C, of the Mannich reaction of
C1–C2
1.422(2)
1.378(2)
1.352(2)
1.499(2)
1.473(2)
123.48(15)
C2–N1–C5
C5–C6–N2
H1ꢁ ꢁ ꢁO1
109.88(14)
116.25(13)
2.001(19)
2.8660(19)
161.3(17)
pyrrole-2-carbaldehyde was explored with piperazine, giving two
new dialdehyde molecules. The [1+2] Schiff base 3, representing
a new extended bis(iminopyrrolyl) ligand containing one piper-
azine ring, was synthesized in high yield. It readily afforded the
three different ‘well-separated’ binuclear five-coordinate copper
(II) complexes containing labile coordinated solvents. Their X-ray
structures showed that both the piperazine nitrogens act as hydro-
gen bond acceptors, instead of coordination to the metal atom.
Other metal complexation reactions including heterobimetallic
complexes and catalytic properties will be our future research.
C2–N1
N1–C5
C5–C6
C6–N2
C1–C2–N1
N1ꢁ ꢁ ꢁO1
N1–H1ꢁ ꢁ ꢁO1
3
N1–C13
C13–C14
C14–N2
N2–C17
C17–C18
1.274(3)
1.432(4)
1.377(3)
1.362(3)
1.488(4)
C18–N3
1.458(3)
121.4(3)
110.5(2)
112.7(2)
N1–C13–C14
C14–N2–C17
C17–C18–N3
6
N1–Cu1
N2–Cu1
Cu1–O1
Cu1–O2
2.018(4)
N1–Cu1–O1
N1–Cu1–N2
H2ꢁ ꢁ ꢁN3
93.72(16)
83.51(16)
1.89(7)
2.638(6)
163(7)
1.92
4
. Experimental
1.948(4)
1.944(4)
2.691(4)
1.928(4)
159.70(17)
170.82(15)
91.19(17)
94.40(17)
4.1. General
O3ꢁ ꢁ ꢁN3
Cu1–O3
O3–H2ꢁ ꢁ ꢁN3
H5aꢁ ꢁ ꢁO4
N1–Cu1–O3
N2–Cu1–O1
O1–Cu1–O3
N2–Cu1–O3
All the reactions and manipulations were carried out in an open
O5ꢁ ꢁ ꢁO4
2.718(5)
163
atmosphere or stated otherwise. Solvents were purchased from
commercial sources and distilled prior to use by following stan-
dard procedures. Pyrrole-2-carbaldehyde was prepared as reported
O5–H5aꢁ ꢁ ꢁO4
7
[
17]. Pyrrole and CDCl
3
were used as received from Aldrich. Other
N1–Cu1
N2–Cu1
Cu1–O1
Cu1–O3
2.051(3)
1.964(3)
1.944(2)
2.258(3)
O(1)–Cu(1)–O(3)
N(2)–Cu(1)–O(3)
O(4)–Cu(1)–O(3)
N(1)–Cu(1)–O(3)
H1ꢁ ꢁ ꢁO2
92.27(11)
91.83(11)
103.40(12)
104.82(11)
1.83(4)
2.592(4)
167(5)
1.93(5)
chemicals were obtained from local commercial sources and were
1
13
used without further purification. H NMR (200 MHz), C NMR
51.3 MHz) and Dept-135 NMR (51.3 MHz) spectra were recorded
(
Cu1–O4
2.013(3)
on Bruker spectrometers. Chemical shifts are referenced with
respect to the chemical shift of the residual protons present in
the deuterated solvents. FTIR spectra were recorded using Perkin
Elmer Spectrum Rx. High Resolution Mass Spectra (ESI) were
recorded using LCT Orthogonal Acceleration TOF Electrospray Mass
O1–Cu1–N2
O(1)–Cu(1)–O(4)
N(2)–Cu(1)–O(4)
O(1)–Cu(1)–N(1)
N(2)–Cu(1)–N(1)
O(4)–Cu(1)–N(1)
174.10(12)
90.05(11)
93.13(12)
92.42(11)
82.42(12)
151.54(13)
O3ꢁ ꢁ ꢁO2
O3–H1ꢁ ꢁ ꢁO2
H2ꢁ ꢁ ꢁN3
O4ꢁ ꢁ ꢁN3
2.700(4)
173(5)
O4–H2ꢁ ꢁ ꢁN3
Spectrometer. Elemental analyses were carried out using
a
8
Perkin–Elmer 2400 CHN analyzer. UV–Vis spectra were recorded
on SHIMADZU UV-2450 spectrophotometer. Melting points were
determined in open capillaries and are corrected using benzophe-
none as a reference.
N1–Cu1
N2–Cu1
Cu1–O1
Cu1–O3
2.035(3)
1.944(3)
1.923(2)
1.997(2)
2.296(2)
147.44(10)
174.23(10)
91.09(9)
93.00(10)
N1–Cu1–O1
N1–Cu1–N2
O3–Cu1–O4
N1–Cu1–O4
H1ꢁ ꢁ ꢁN3
91.52(10)
82.85(11)
100.48(9)
111.66(10)
1.99(4)
2.658(3)
177(5)
1.79(4)
Cu1–O4
N1–Cu1–O3
N2–Cu1–O1
O1–Cu1–O3
N2–Cu1–O3
O3ꢁ ꢁ ꢁN3
0
4.2. Synthesis of N,N -bis(5-formylpyrrol-1-ylmethyl)piperazine 1
O3–H1ꢁ ꢁ ꢁN3
H2ꢁ ꢁ ꢁO2
O4ꢁ ꢁ ꢁO2
2.609(3)
168(4)
To
a suspension of piperazine (0.43 g, 5.00 mmol) and
O4–H2ꢁ ꢁ ꢁO2
formaldehyde (38%, 0.80 mL, 10.00 mmol) in methanol/water
(
20 ml, v/v 2:1) was added a solution of pyrrole-2-carbaldehyde
H
N
N
H
H
H
N
N
N
N
H
N
N
N
H
H
H
N
O
N
4
, 20%
3, 53%
(
i) HNO
3 3
/r.t./4 h/MeOH/CHCl
2
(
ii) NaOH/30 min
H
NH
2
N
1
H
N
(
(
i) HNO /r.t./4 h/MeOH
3
ii) NaOH/30 min
5, 58%
Scheme 2. The Schiff base condensation reactions for the dialdehydes 1 and 2: synthesis of the bis(iminopyrrolyl) 3 and monoiminopyrrolyl 4 ligands containing the
piperazine ring.

7
4
R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
Fig. 2. The X-ray crystal structure of the Schiff base 3. Symmetry transformations used to generate equivalent atoms: ꢀx + 1, ꢀy + 1, ꢀz.
H
N
H
N
N
N
H
N
N
H
3
2
[Cu(OOCH)
2
(H
2
O)]
2 [Cu(OOR)
2
(H
2
O)]
HO
H
O
MeOH/r.t./2 h
MeOH/r.t./2 h
O
OH
Cu
O
R
S
O
N
Cu
N
N
N
H
N
H
N
H
H
N
N
N
N
N
N
Cu
O
R = Me, 7 (78%)
R = Ph, 8 (77%)
Cu
6, (81%)
OH
O
OH2
R
HO
O
O
H
Scheme 3. Synthesis of the binuclear copper(II) complex 6–8 containing the bis(iminopyrrolyl) ligand 3.
Fig. 3. The X-ray crystal structure of the dinuclear Cu(II) formate complex 6. Most H atoms and lattice MeOH are omitted for clarity. Dotted lines indicate hydrogen bonding.
Symmetry transformations used to generate equivalent atoms: (i) ꢀx + 1, ꢀy + 1, ꢀz + 1, (ii) ꢀx + 1, ꢀy + 1, ꢀz and (iii) ꢀx, ꢀy + 1, ꢀz.
1
(
0.95 g, 10.00 mmol) in methanol (5 mL) and the solution was stir-
CH and pyrrole
CDCl , 25 °C, ppm): d = 49.9 (N(CH
109.9 (pyrrole b-C), 125.4 (pyrrole b-C), 132.1 (pyrrole
a
-CH), 9.52 (s, 2H, CHO). 13C{ H}NMR (51.3 MHz,
red overnight (15 h) at room temperature. The solvent was
removed under vacuum and the resulting residue was extracted
with chloroform (2 ꢂ 15 mL). The chloroform solution was dried
over anhydrous sodium sulfate for two hours and then filtered.
The solvent was removed under vacuum to give compound 1 as
oily residue which became solid over a period of three hours
(
2
3
2
)
4
N), 68.8 (CH
2
N(CH
2
)
4
NCH
-C),
-C), 179.9 (CHO). DEPT-135{ H}NMR (51.3 MHz,
, 25 °C, ppm): d = 49.7 (N(CH N), 68.6 (CH N(CH NCH ),
109.7 (pyrrole b-C), 125.2 (pyrrole b-C), 131.9 (pyrrole -C),
= 3296 (w), 3104 (w), 2948 (w),
2
),
a
1
132.5 (pyrrole
CDCl
a
3
2
)
4
2
2
)
4
2
a
ꢀ1
179.8 (CHO). FT-IR (KBr, cm ): m
1
1.06 g, 3.53 mmol, 71%). mp 96 °C. H NMR (200 MHz, CDCl
3
,
2917 (w), 2884 (w), 2835 (m), 2717 (w), 1657 (vs), 1525 (w),
1473 (m), 1455 (m), 1422 (w), 1403 (vs), 1372 (s), 1353 (s), 1328
(m), 1287 (s), 1209 (w), 1164 (vs), 1133 (m), 1079 (s), 1067 (m),
5 °C, ppm): d = 2.56 (s, 8H, N(CH
2 4 2 2 4
) N), 5.15 (s, 4H, CH N(CH ) -
NCH ), 6.23 (m, 2H, pyrrole b-CH), 6.97–6.92 (m, 4H, pyrrole b-
2

R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
75
Fig. 4. The X-ray crystal structure of the dinuclear Cu(II) acetate complex 7. Most H atoms are omitted for clarity. Dotted lines indicate hydrogen bonding. Symmetry
transformations used to generate equivalent atoms: (i) ꢀx + 1, ꢀy, ꢀz + 1 and (ii) ꢀx + 1, y + 1/2, ꢀz + 1/2.
Fig. 5. The X-ray crystal structure of the dinuclear Cu(II) benzoate complex 8. Most H atoms and lattice MeOH are omitted for clarity. Dotted lines indicate hydrogen bonding.
Symmetry transformations used to generate equivalent atoms: (i) ꢀx + 1, ꢀy, ꢀz + 1 and (ii) ꢀx + 3/2, ꢀy + 3/2, z.
1
6
3
036 (m), 1011 (s), 881 (w), 793 (w), 767 (vs), 747 (vs), 650 (m),
1416 (m), 1359 (w), 1339 (m), 1273 (w), 1190 (s), 1141 (w),
+
09 (m), 497 (w). HRMS(+ESI): calc. m/z for [M+H ] C16
21
H N
4
O
2
:
1055 (m), 1013 (w), 886 (w), 818 (m), 766 (m), 642 (w), 424 (w).
+
01.1665, found: 301.1626.
HRMS(+ESI): calc. m/z for [M+H ] C16
H
21
N
4
2
O : 301.1665, found:
3
01.1626.
0
4
.3. Synthesis of N,N -bis(5-formylpyrrol-2-ylmethyl)piperazine, 2
4.4. Synthesis of bis(iminopyrrolyl), 3 and monoiminopyrrolyl, 4
To
formaldehyde (38%, 0.80 mL, 10.00 mmol) in methanol/water
20 mL, v/v 2:1) was added conc. HCl (12 N, 0.83 mL, 10.00 mmol)
at room temperature. The reaction mixture turned to a homoge-
neous solution. A solution of pyrrole-2-carbaldehyde (0.95 g,
a
suspension of piperazine (0.43 g, 5.00 mmol) and
ligands
(
To a solution of 2 (0.92 g, 3.06 mmol) and 2,6-diisopropylaniline
(1.21 mL, 6.41 mmol) in methanol/chloroform (30 mL, v/v 2:3) was
added conc. HNO (16 N, 0.40 mL, 6.41 mmol), resulting in an
3
1
0.00 mmol) in methanol (5 mL) was added and the solution was
stirred overnight (15 h) to give colorless precipitate. Solid NaOH
0.40 g, 10.00 mmol) was then added and the resulting reaction
immediate formation of colorless precipitate. The solution was
stirred for 4 h at room temperature. Solid NaOH (0.26 g,
6.41 mmol) was then added and stirred for an additional 0.5 h.
The solvent was removed under vacuum and the resulting residue
was extracted with dichloromethane which was loaded onto a col-
umn chromatography filled with silica gel. Elution using ethyl
(
mixture was stirred for one hour. Methanol was partially removed
under vacuum and the reaction mixture was filtered. The precipi-
tate was washed with water (2 ꢂ 5 mL) and dried under vacuum
to give 2 (1.06 g, 3.53 mmol, 71%) as colorless solid. mp > 200 °C.
acetate/petroleum ether (v/v 1:2) afforded the first fraction from
1
H NMR (200 MHz, CDCl
.56 (s, 4H, CH N(CH NCH
b-CH), 6.89 (t, 2H, J(H,H) = 4 Hz, pyrrole b-CH), 9.43 (s, 2H, CHO),
3
, 25 °C, ppm): d = 2.50 (s, 8H, N(CH
2
)
4
N),
which the solvents were removed under vacuum to give Schiff base
3 (1.01 g, 1.63 mmol, 53%) as colorless solid. Further elution using
the same solvent mixture yielded the second fraction from which
the solvents were removed under vacuum to give the mono Schiff
3
2
2
)
4
2
), 6.16 (t, 2H, 3J(H,H) = 4 Hz, pyrrole
3
1
3
1
9
.61 (br s, 2H, NH). C{ H} NMR (51.3 MHz, CDCl
d = 53.3 (N(CH N), 55.2 (CH N(CH NCH ), 110.6 (pyrrole b-C),
21.9 (pyrrole b-C), 132.7 (pyrrole -C), 138.2 (pyrrole -C),
78.9 (CHO). DEPT-135{ H} NMR (51.3 MHz, CDCl , 25 °C, ppm):
N), 54.9 (CH N(CH NCH ), 110.4 (pyrrole b-C),
21.7 (pyrrole b-C), 178.6 (CHO). FT-IR (KBr, cm ):
940 (w), 2826 (m), 1642 (vs), 1560 (w), 1490 (m), 1460 (w),
3
, 25 °C, ppm):
2
)
4
2
2
)
4
2
base 4 (0.280 g, 0.61 mmol, 20%) as colorless solid.
1
1
1
a
a
For 3: mp 166 °C. H NMR (200 MHz, CDCl
3
, 25 °C, ppm):
), 2.65 (s, 8H, N(CH N),
2.97–3.11 (m, 2H, CH), 3.65 (s, 4H, CH –N(CH N–CH ), 6.18 (d,
1
3
3
d = 1.17 (d, 24H, J(H,H) = 7 Hz, CH
3
2 4
)
d = 53.0 (N(CH
1
2
2
)
4
2
2
)
4
2
2
2
)
4
2
ꢀ1
3
3
m
= 3239 (s),
2H, J(H,H) = 3.4 Hz, pyrrole b-CH), 6.55 (d, 2H, J(H,H) = 3.2 Hz,
pyrrole b-CH), 7.13–7.16 (m, 6H, benzene CH), 7.85 (s, 2H,

7
6
R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
HC = N). 13C{ H} NMR (51.3 MHz, CDCl
CH ), 28.0 (CH), 53.3 (N(CH N), 55.5 (CH
09.9 (pyrrole b-C), 116.3 (pyrrole b-C), 123.2 (benzene-C), 124.2
benzene-C), 130.2 (pyrrole -C), 134.5 (pyrrole -C), 138.6 (ben-
zene-C), 149.0 (benzene-C), 151.8 (HC@N). DEPT-135{ H} NMR
1
, 25 °C, ppm): d = 23.9
–N(CH N–CH ),
674 (w), 571 (w), 451 (w). Anal. Calc. for C42
57.98; H, 6.72; N, 9.66. Found: C, 57.61; H, 6.87; N, 9.86%.
3
58 6 6 2
H N O Cu : C,
(
1
(
3
2
)
4
2
2
)
4
2
a
a
4.7. Synthesis of the binuclear copper(II) acetate complex, 7
1
(
(
(
51.3 MHz, CDCl
CH N), 55.5 (CH
pyrrole b-C), 123.2 (benzene-C), 124.2 (benzene-C), 151.7 (HC@N).
3
, 25 °C, ppm): d = 23.9 (CH
3
), 28.0 (CH), 53.3 (N
To a solution of [Cu(OAc) (H O)] (0.040 g, 0.2 mmol) in metha-
2
2
)
2 4
2
–N(CH N–CH ), 109.8 (pyrrole b-C), 116.3
)
2 4
2
nol (8 mL) was added solid 3 (0.062 g, 0.1 mmol) and the color of
the solution immediately changed to green. The solution was stir-
red at room temperature for 2 h, giving a little amount of precipi-
tation. Dichloromethane (3 mL) was then added and the solution
became a homogeneous solution, which yielded dark green crys-
tals of 7 upon slow evaporation of the solvents at room tempera-
ꢀ
1
FT-IR (KBr, cm ):
m
= 3446 (m), 2958 (m), 2923 (w), 2821 (w),
2
1
1
364 (w), 1720 (w), 1627 (s), 1588 (w), 1562 (w), 1499 (w),
458 (w), 1437 (w), 1385 (w), 1335 (w), 1294 (w), 1232 (w),
159 (m), 1134 (w), 1051 (w), 1009 (w), 930 (w), 857 (w), 830
(
w), 793 (m), 749 (w), 718 (w), 677 (w), 578 (w). HRMS(+ESI): calc.
ture. Yield: 0.072 g, 0.075 mmol, 75% based on 7ꢁ2H
2
O. Mp
+
m/z for [M+H ] C40
H
55
N
6
: 619.4488, found: 619.4512.
ꢀ1
1
1
1
6
7
68 °C. FT-IR (KBr, cm ):
m
= 3434 (w), 2962 (m), 1578 (vs),
1
For 4: mp 174 °C. H NMR (200 MHz, CDCl
3
, 25 °C, ppm):
), 2.60 (s, 8H, N(CH N),
431 (m), 1384 (m), 1308 (m), 1286 (m), 1260 (w), 1178 (w),
052 (m), 992 (w), 933 (w), 894 (w), 825 (w), 757 (w), 732 (w),
3
d = 1.17 (d, 12H, J(H,H) = 6.8 Hz, CH
2
CH
6
=
1
3
2 4
)
2 2 4 2
.96–3.10 (m, 2H, CH), 3.60 (s, 2H, CH N(CH ) NCH ), 3.64 (s, 2H,
74 (w), 617 (w). Anal. Calc. for C46
70 6 8 2
H N O Cu : C, 57.42; H,
N(CH
2
)
4
NCH
2
), 6.17 (d, 2H, 3J(H,H) = 3.2 Hz, pyrrole b-CH),
2
.33; N, 8.73. Found: C, 57.97; H, 7.91; N, 8.90%.
3
3
.57 (d, 1H, J(H,H) = 3.2 Hz, pyrrole b-CH), 6.88 (d, 1H, J(H,H)
3.6 Hz, pyrrole b-CH), 7.02–7.18 (m, 3H, benzene CH), 7.84 (s,
H, HC@N), 9.44 (s, 1H, CHO), 9.82 (br s, 1H, NH). 1 C{ H} NMR
3
1
4.8. Synthesis of the binuclear copper(II) benzoate complex, 8
To a suspension of [Cu(OOCPh) (H O)] (0.065 g, 0.2 mmol) in
(
51.3 MHz, CDCl
3 3
, 25 °C, ppm): d = 23.9 (CH ), 28.1 (CH), 53.0 (N
2
2
(CH N), 55.0 (CH
)
2 4
2
N(CH NCH ), 110.4 (pyrrole b-C), 110.8 (pyr-
2
)
4
2
methanol (10 mL) was added solid 3 (0.062 g, 0.1 mmol) and the
color of the solution immediately changed to green. The solution
was stirred at room temperature for 2 h, resulting in the precipi-
tate formation. Dichloromethane (10 mL) was then added and
the solution became a homogeneous solution, which yielded dark
green crystals of 8 upon slow evaporation of the solvents at room
temperature. Yield: 0.089 g, 0.077 mmol, 77% based on 8ꢁMeOH.
role b-C), 121.8 (pyrrole b-C), 123.3 (benzene-C), 124.6 (pyrrole
b-C), 132.8 (pyrrole -C), 139.0 (pyrrole -C), 151.9 (HC@N),
78.9 (CHO). DEPT-135{ H} NMR (51.3 MHz, CDCl , 25 °C, ppm):
N), 55.1 (CH –N(CH N–
a
a
1
1
3
d = 23.7 (CH
3
), 27.9 (CH), 52.8 (N(CH
2
)
4
2
2 4
)
2
CH ), 110.2 (pyrrole b-C), 110.6 (pyrrole b-C), 121.7 (pyrrole b-C),
1
23.1 (benzene-C), 124.5 (pyrrole b-C), 151.8 (HC@N), 178.7
ꢀ1
(
(
(
CHO). FT-IR (KBr, cm ):
s), 1439 (m), 1342 (m), 1169 (m), 1040 (m), 784 (m). HRMS
+ESI): calc. m/z for [M+H ]
m = 3255 (m), 2957 (m), 2813 (w), 1634
ꢀ
1
Mp 174 °C. FT-IR (KBr, cm ):
2826 (w), 1597 (vs), 1575 (vs), 1460 (m), 1446 (m), 1377 (vs),
307 (s), 1283 (s), 1257 (m), 1176 (m), 1139 (w), 1054 (vs), 1025
w), 991 (w), 933 (w), 895 (w), 823 (w), 805 (w), 758 (m), 718
s), 681 (m), 572 (w), 456 (w). Anal. Calc. for C58 Cu : C,
2.51; H, 7.06; N, 7.54. Found: C, 62.32; H, 7.04; N, 7.98%.
m = 3063 (w), 2961 (s), 2868 (m),
+
28 38 5 1
C H N O : 460.3076, found:
1
4
60.3111.
(
(
H
78
N
6
O
8
2
4.5. Synthesis of iminopyrrole 5
6
To a solution of 1 (0.205 g, 0.68 mmol) and 2,6-diisopropylani-
line (0.26 mL, 1.37 mmol) in methanol/chloroform (30 mL, v/v 2:3)
was added conc. HNO (16 N, 0.085 mL, 1.37 mmol), giving a clear
solution. After stirring the solution for 21 h at room temperature,
solid NaOH (0.055 g, 1.37 mmol) was added and stirring continued
for an additional 0.5 h. The solvent was removed under vacuum
and the resulting residue was extracted with dichloromethane,
which was loaded onto a column chromatography filled with silica
4.9. X-ray crystallography
3
Suitable single crystals of 2 and 3 for X-ray diffraction studies
were obtained by slow evaporation of a solution of 2 and 3 in
petroleum ether/dichloromethane (1:1 v/
evaporation of a solution of 6–8 in methanol/dichloromethane
(2:1 v/ ) gave suitable single crystals of 6–8.
v). Similarly, the slow
v
gel. Elution using ethyl acetate/petroleum ether (
the first fraction from which the solvents were removed under vac-
v
/v 1:5) afforded
Single crystal X-ray diffraction data collections for all the com-
pounds were performed using Bruker-APEX-II CCD diffractometer
uum to give Schiff base 5 (0.20 g, 0.79 mmol, 58%) as colorless
with graphite monochromated Mo Ka radiation (k = 0.71073 Å).
1
13
solid. The structure of 5 is confirmed by matching the H and
C
The space group for every structure was obtained by XPREP program.
The structures were then solved by SIR-92 [18] or SHELXS-97 [19]
available in WinGX, which successfully located most of the non-
hydrogen atoms. Subsequently, least square refinements were car-
NMR spectra with the reported ones.
4.6. Synthesis of the binuclear copper(II) formate complex, 6
2
ried out on F using SHELXL-97 (WinGX version) to locate the
To a solution of [Cu(OOCH) (H O)] (0.035 g, 0.2 mmol) in
2
2
remaining non-hydrogen atoms. Non-hydrogen atoms were
refined with anisotropic displacement parameters. The pyrrolic
NH, methanol OH and water hydrogens were located from the
difference Fourier maps; all other hydrogen atoms were placed at
calculated positions and refined using a riding model with fixed
isotropic displacement parameters that were 1.2–1.5 times the
equivalent isotropic displacement parameter for the parent atom
to which a given hydrogen atom was bonded. The formate complex
6 crystallizes in the centrosymmetric triclinic space group P1. The
asymmetric unit consists of one half of the molecule and two
MeOH molecules. The acetate complex 7 crystallizes in the cen-
trosymmetric monoclinic space group P2 /c. The asymmetric unit
1
methanol (10 mL) was added solid 3 (0.062 g, 0.1 mmol) and the
color of the solution immediately changed to green. The solution
was stirred at room temperature for 2 h, giving a little amount of
precipitation. Dichloromethane (10 mL) was then added and the
solution became a homogeneous solution, which yielded dark
green crystals of 6 upon slow evaporation of the solvents at room
temperature. These crystals become amorphous immediately in
the absence of mother liquor. Yield: 0.071 g, 0.081 mmol, 81%
without 4MeOH solvents of crystallization). Mp > 200 °C. FT-IR
ꢀ
ꢀ
1
(
(
(
KBr, cm ):
m
= 2961 (s), 2868 (m), 2832 (m), 1609 (vs), 1570
vs), 1502 (w), 1476 (m), 1459 (m), 1418 (w), 1379 (m), 1347
m), 1305 (m), 1284 (s), 1258 (m), 1176 (m), 1116 (w), 1055 (s),
contains one half of the molecule. The structure of the benzoate
9
90 (w), 933 (w), 895 (w), 825 (w), 778 (m), 761 (m), 734 (w),
complex 8 was solved in the centrosymmetric orthorhombic Pccn

R. Kumar et al. / Inorganica Chimica Acta 445 (2016) 70–78
77
Table 2
Crystal data and structure refinement for 2, 3 and 6–8.
2
3
6ꢁ4MeOH
74Cu
7
8ꢁMeOH
2 6 9
82Cu N O
Empirical formula
Formula weight
Wavelength (Å)
T (K)
Crystal system
Color and shape
Space group
a (Å)
C
16
H
20
N
4
O
2
C
40
H
54
N
6
C
46
H
2
N
6
O
10
C
48
H
2
74Cu N
6
O
8
C
59
H
300.36
0.71073
293(2)
triclinic
colorless
P1ꢀ
618.89
0.71073
293(2)
triclinic
colorless
P1ꢀ
6.381(3)
7.405(4)
20.224(10)
99.299(16)
95.148(16)
100.748(16)
919.3(8)
1
998.19
0.71073
100(2)
triclinic
green
P1ꢀ
9.270(3)
11.152(4)
13.282(5)
77.375(11)
75.184(11)
77.569(12)
1276.7(8)
1
990.21
0.71073
100(2)
monoclinic
green
1146.38
0.71073
100(2)
orthorhombic
green
1
P2 /c
Pccn
5.3756(6)
7.7191(9)
16.647(3)
9.6898(19)
16.057(3)
90
94.725(6)
90
2581.3(9)
2
1.274
35.432(3)
10.9934(10)
14.9308(13)
90
90
90
5815.8(9)
4
1.309
b (Å)
c (Å)
10.2528(16)
107.841(4)
92.989(4)
109.053(3)
377.28(8)
1
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
l
calc (g cmꢀ3)
1.322
0.090
1.118
0.066
1.298
0.891
ꢀ
1
(mm
)
0.878
0.791
F(000)
160
336
530
1052
2432
h range (°)
Limiting indices
2.117–24.996
1.028–24.994
1.610–25.000
2.434–24.999
ꢀ19 6 h 6 19,
ꢀ10 6 k 6 11,
ꢀ19 6 l 6 19
28643/4515
1.149–24.962
ꢀ42 6 h 6 42,
ꢀ13 6 k 6 13,
ꢀ17 6 l 6 15
64863/5084
ꢀ6 6 h 6 6, ꢀ9 6 k 6 9, ꢀ7 6 h 6 7, ꢀ8 6 k 6 8, ꢀ11 6 h 6 11,
ꢀ
12 6 l 6 12
ꢀ23 6 l 6 23
ꢀ13 6 k 6 10,
ꢀ
15 6 l 6 14
Total/unique no. of
reflns.
4387/1320
10937/3209
13220/4292
R
int
0.0214
0.0732
0.0623
0.1033
0.0877
Data/restr./params.
1320/0/103
3209/0/211
1.004
4292/0/299
1.116
4515/0/295
1.085
5084/0/355
1.007
Goodness-of-fit (GOF) on 1.081
2
F
R
1
, wR
R indices (all data) R
wR
Largest different peak
2
0.0387, 0.1023
0.0468, 0.1076
0.0569, 0.1277
0.1328, 0.1740
0.0586, 0.1666
0.0869, 0.2049
0.0479, 0.1185
0.0769, 0.1349
0.0465, 0.1288
0.0642, 0.1450
1
,
2
0.137 and ꢀ0.227
0.207 and ꢀ0.265
0.761 and ꢀ1.018
0.424 and ꢀ 0.653
1.011 and ꢀ0.537
ꢀ
3
and hole (e Å
)
space group. The asymmetric unit contains one half of the mole-
cule and one disordered methanol at the special position as solvent
of crystallization. Methanol hydrogens could not be placed at cal-
culated positions or located and refined. The refinement data for
all the structures are summarized in Table 2.
(g) C.S.B. Gomes, M.T. Duarte, P.T. Gomes, J. Organomet. Chem. 760 (2014) 167;
h) R. Li, B. Moubaraki, K.S. Murray, S. Brooker, Dalton Trans. (2008) 6014.
2] (a) R.H. Holm, A. Chakravorty, L.J. Theriot, Inorg. Chem. 5 (1966) 625;
b) D.M. Dawson, D.A. Walker, M. Thornton-Pett, M. Bochmann, J. Chem. Soc.,
Dalton Trans. (2000) 459;
c) M.G. Crestani, G.F. Manbeck, W.W. Brennessel, T.M. McCormick, R.
Eisenberg, Inorg. Chem. 50 (2011) 7172;
d) E.M. Matson, J.A. Bertke, A.R. Fout, Inorg. Chem. 53 (2014) 4450.
(
[
(
(
(
Acknowledgments
[
3] D. Suresh, C.S.B. Gomes, P.S. Lopes, C.A. Figueira, B. Ferreira, P.T. Gomes, R.E.
DiPaolo, A.L. Maçanita, M.T. Duarte, A. Charas, J. Morgado, D. Vila-Viçosa, M.J.
Calhorda, Chem. Eur. J. 21 (2015) 9133.
We thank the CSIR and DST, India for support and for the X-ray
and NMR facilities. We thank Dr. Carola Schulzke, Ernst-Moritz-
Arndt-Universität Greifswald, Institut für Biochemie, Germany for
elemental analysis data.
[
4] (a) V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2003) 283;
(
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